Most fish possess highly developed sense organs. Nearly all fish that are primarily active during daylight have colour vision that is at least as good as a human's. Many fish also have chemoreceptors responsible for extraordinary senses of taste and smell. Their hearing is adapted to the underwater environment, using bone conduction, the swim bladder, and the inner ear. Most fish have sensitive receptors that form the lateral line system, which detects gentle currents and vibrations, and senses the motion of nearby fish and prey. Sharks can sense frequencies in the range of 25 to 50 Hz through their lateral line.
Fish orient themselves using landmarks and may use mental maps based on multiple landmarks or symbols. Fish behavior in mazes reveals that they possess spatial memory and visual discrimination.
Vision
Vision is an important sensory system for most species of fish. Fish eyes are similar to those of terrestrial vertebrates like birds and mammals, but have a more spherical lens. Their retinas generally have both rod cells and cone cells (for scotopic and photopic vision), and most species have colour vision. Some fish can see ultraviolet and some can see polarized light. Amongst jawless fish, the lamprey has well-developed eyes, while the hagfish has only primitive eyespots.N. A. Campbell and J. B. Reece (2005). Biology, Seventh Edition. Benjamin Cummings, San Francisco, California. Fish vision shows adaptation to their visual environment, for example deep sea fishes have eyes suited to the dark environment.
Fish and other aquatic animals live in a different light environment than terrestrial species. Water absorbs light so that with increasing depth the amount of light available decreases quickly. The optic properties of water also lead to different wavelengths of light being absorbed to different degrees, for example light of long wavelengths (e.g. red, orange) is absorbed quite quickly compared to light of short wavelengths (blue, violet), though ultraviolet light (even shorter wavelength than blue) is absorbed quite quickly as well. Besides these universal qualities of water, different bodies of water may absorb light of different wavelengths because of salts and other chemicals in the water.
Hearing, motion, and pressure
Hearing is an important sensory system for most species of fish. For example, in the family Batrachoididae, males use their swim bladders to make advertisement calls which females use to localize males. Hearing threshold and the ability to localize sound sources are reduced underwater, in which sound travels faster than in air. Underwater hearing is by bone conduction, and localization of sound appears to depend on differences in amplitude detected by bone conduction. As such, aquatic animals such as fish have a more specialized hearing apparatus that is effective underwater.
Fish sense sound primarily through their otoliths (inner ears). In some fishes, such as some species of carp and herring, hearing is enhanced by a connection between their swim bladder and inner ear.WILLIAMS, C. "Sense of Hearing in Fishes." Nature 147.3731 (n.d.): 543. The lateral line consists of a series of receptors along the side of a fish's body, open to the environment via a series of openings called lateral line pores. The lateral line can also play a role in hearing.
Fishes can be characterised into six hearing groups for assessing their susceptibility to underwater sound, depending on the involvement of the swim bladder in hearing. For three groups (denoted P1, P2 and P3), the swim bladder is involved in hearing, which is thus sensitive to sound pressure. For the other three groups (A2, A3 and A4), the swim bladder is either not present or not involved in hearing and the hearing responds not to sound pressure but to sound particle acceleration.Lucke, K., MacGillivray, A. O., Halvorsen, M. B., Ainslie, M. A., Zeddies, D. G., & Sisneros, J. A. (2024). Recommendations on bioacoustical metrics relevant for regulating exposure to anthropogenic underwater sound. The Journal of the Acoustical Society of America, 156(4), 2508-2526.
Fish hearing groups sensitive to sound pressure are P1 (swim bladder without adaptation such as the cod or haddock), P2 (swim bladder with a mechanical connection to the inner ear such as the Weberian ossicle), e.g., goldfish and zebrafish and P3 (swim bladder with ultrasonic adaptation, bubble near ear, or a standard ear such as shad, menhaden). Hearing group A4 comprises bony fishes with a swim bladder not involved in hearing such as sturgeon and salmon. Fish hearing groups without a swim bladder are A2 (cartilaginous fishes such as shark and skate) and A3 (bony fishes without a swim bladder, such as dab and mackerel).
Pressure detection uses the organ of Weber, a system consisting of three appendages of vertebrae transferring changes in shape of the gas bladder to the middle ear. It can be used to regulate the buoyancy of the fish. Fish like the weather fish and other loaches are also known to respond to low pressure areas but they lack a swim bladder.
Current detection
The lateral line in fish and aquatic forms of amphibians is a detection system of water currents, consisting mostly of vortices. The lateral line is also sensitive to low-frequency vibrations. It is used primarily for navigation, hunting, and schooling. The mechanoreceptors are hair cells, the same mechanoreceptors for vestibular sense and hearing. Hair cells in fish are used to detect water movements around their bodies. These hair cells are embedded in a jelly-like protrusion called cupula. The hair cells therefore can not be seen and do not appear on the surface of skin. The receptors of the electrical sense are modified hair cells of the lateral line system.
Fish and some aquatic amphibians detect hydrodynamic stimuli via a lateral line. This system consists of an array of sensors called neuromasts along the length of the fish's body.Bleckmann, H, and R Zelick. "Lateral line system of fish." Integrative Zoology 4 (2009): 13-25. . Neuromasts can be free-standing (superficial neuromasts) or within fluid-filled canals (canal neuromasts). The sensory cells within neuromasts are polarized hair cells contained within a gelatinous cupula.Herring, Peter. The Biology of the Deep Ocean. New York: Oxford, 2002. The cupula, and the stereocilia which are the "hairs" of hair cells, are moved by a certain amount depending on the movement of the surrounding water. Afferent nerve fibers are excited or inhibited depending on whether the hair cells they arise from are deflected in the preferred or opposite direction. Lateral line neurons form somatotopic maps within the brain informing the fish of amplitude and direction of flow at different points along the body. These maps are located in the medial octavolateral nucleus (MON) of the medulla and in higher areas such as the torus semicircularis.
Chemoreception (smelling)
The aquatic equivalent to smelling in air is tasting in water. Many larger catfish have chemoreceptors across their entire bodies, which means they "taste" anything they touch and "smell" any chemicals in the water. "In catfish, gustation plays a primary role in the orientation and location of food".Atema, Jelle (1980) "Chemical senses, chemical signals, and feeding behavior in fishes" pp. 57–101. In: Bardach, JE Fish behavior and its use in the capture and culture of fishes, The WorldFish Center, .
Salmon have a strong sense of smell. Speculation about whether odours provide homing cues, go back to the 19th century.Trevanius 1822 In 1951, Hasler hypothesised that, once in vicinity of the estuary or entrance to its birth river, salmon may use chemical cues which they can smell, and which are unique to their natal stream, as a mechanism to home onto the entrance of the stream.Hasler 1951 In 1978, Hasler and his students convincingly showed that the way salmon locate their home rivers with such precision was indeed because they could recognise its characteristic smell. They further demonstrated that the smell of their river becomes imprinted in salmon when they transform into smolts, just before they migrate out to sea.Hasler 1978Dittman 1996 Homecoming salmon can also recognise characteristic smells in tributary streams as they move up the main river. They may also be sensitive to characteristic pheromones given off by juvenile conspecifics. There is evidence that they can "discriminate between two populations of their own species".Groot 1986
Sharks have keen olfactory senses, located in the short duct (which is not fused, unlike bony fish) between the anterior and posterior nasal openings, with some species able to detect as little as one part per million of blood in seawater. Sharks have the ability to determine the direction of a given scent based on the timing of scent detection in each nostril.The Function of Bilateral Odor Arrival Time Differences in Olfactory Orientation of Sharks , Jayne M. Gardiner, Jelle Atema, Current Biology - 13 July 2010 (Vol. 20, Issue 13, pp. 1187-1191) This is similar to the method mammals use to determine the direction of sound. They are more attracted to the chemicals found in the intestines of many species, and as a result often linger near or in sewage outfalls. Some species, such as nurse sharks, have external barbels that greatly increase their ability to sense prey.
The MHC genes are a group of genes present in many animals and important for the immune system; in general, offspring from parents with differing MHC genes have a stronger immune system. Fish are able to smell some aspect of the MHC genes of potential sex partners and prefer partners with MHC genes different from their own.
Electroreception and magnetoreception
Electroreception is the ability to detect electric fields or currents. Some fish, such as catfish and sharks, have organs that detect weak electric potentials on the order of millivolts.Albert, J.S., and W.G.R. Crampton. 2005. Electroreception and electrogenesis. pp. 431–472 in The Physiology of Fishes, 3rd Edition. D.H. Evans and J.B. Claiborne (eds.). CRC Press. Other fish, like the South American electric fishes Gymnotiformes, can produce weak electric currents, which they use in navigation and social communication. In sharks, the ampullae of Lorenzini are electroreceptor organs. They number in the hundreds to thousands. Sharks use the ampullae of Lorenzini to detect the electromagnetic fields that all living things produce. This helps sharks (particularly the hammerhead shark) find prey. The shark has the greatest electrical sensitivity of any animal. Sharks find prey hidden in sand by detecting the electric fields they produce. Ocean currents moving in the magnetic field of the Earth also generate electric fields that sharks can use for orientation and possibly navigation. Among teleosts, the electric catfish uses electroreception to navigate through muddy waters. These fish make use of spectral changes and amplitude modulation to determine factors such shape, size, distance, velocity, and conductivity. The abilities of the electric fish to communicate and identify sex, age, and hierarchy within the species are also made possible through electric fields. EF gradients as low as 5 nV/cm can be found in some saltwater weakly electric fish.Zimmerman, T., Smith, J., Paradiso, J., Allport, D., & Gershenfeld, N. (1995). Applying Electric Field Sensing to Human-Computer Interfaces. IEEE SIG . Several basal bony fishes, including the paddlefish (Polyodon spathula), possess electroreceptors. The paddlefish hunts plankton using thousands of tiny passive electroreceptors located on its extended snout, or rostrum. The paddlefish is able to detect electric fields that oscillate at 0.5–20 Hz, and large groups of plankton generate this type of signal.Montgomery JC, Coombs S, Baker CF (2001) The mechanosensory lateral line system of the hypogean form of Astyanax fasciatus. Env Biol Fish 62:87–96